The present invention relates generally to multiplex communications systems used, for example, for motor vehicles and, more particularly, to driver circuits for connecting individual nodes of a multiplex communications system with a two wire, preferably twisted pair, communications bus which interconnects nodes of the system.
The use of multiplex communications systems in motor vehicles has been known for some time to reduce the large number of wires which are used within vehicles. In addition, such multiplex systems benefit the design, assembly and diagnostics of motor vehicles. A common component of multiplex systems is a bus circuit which interconnects nodes of a given system. Since a bus circuit typically extends throughout a motor vehicle, it can reach a substantial length and, accordingly, multiplex system buses are preferably constructed as simply as possible to reduce the costs of the multiplex system. One popular bus circuit is constructed using a twisted pair of wires which are terminated at least at one point to ground for one wire and to system signal potential for the other wire.
In the past, driver circuits for connecting individual nodes of a motor vehicle multiplex communications system to a bus circuit have been problematic. Known bus drivers often include a substantial number of components and hence can be costly and occupy a substantial amount of space. In addition, reliability has plagued some bus drivers. For example, multiplex communications systems should not be disabled if a single node loses its power or ground, the bus or one side of the bus is shorted to power or ground, or the bus or one side of the bus is shorted to reverse power. Many known bus drivers cannot survive such mishaps. In the motor vehicle environment, it is even possible for a bus driver to be subjected to double battery voltage or reversed double battery voltage by attempts to jump-start the vehicle.
Accordingly, there is a need for a low cost bus driver circuit which can reliably drive a bus circuit of a multiplex communications system of a motor vehicle. The bus driver circuit should preclude interference with other nodes communicating over the bus in the event one or more nodes each including the bus driver circuit are disabled by loss of power or ground. Preferably, such a bus driver circuit would be able to withstand bus shorts to ground, bus shorts to vehicle power, or even bus shorts to direct or reversed double vehicle power without sustaining damage.